In recent years, outbreaks of healthcare-associated infections resulting from toxin-producing Clostridium difficile (C. difficile) have been widespread and are associated with increased morbidity and mortality. C. difficile is responsible for 90-100% of cases of pseudomembranous colitis, 15-25% of antibiotic-associated diarrheas and 60-75% of antibiotic-associated colitis.1-4 According to a recent Canadian study, the incidence of C. difficile infection (CDI) in Canada is 65 CDI cases per 100,000 patient-days for adult patients admitted to hospitals, accompanied with a mortality rate of 5.7%.5 In the United States, about 15,000-20,000 patients die annually from CDI.6 Furthermore, the economic burden for taking care of CDI patients is significant: a hospital-acquired CDI increases the cost of otherwise matched hospitalizations by 4-fold, translating to over $1 billion in added cost annually.7 A key to reducing the negative outcomes of CDI is rapid and accurate diagnosis so that the treatment of patients can be quickly implemented and the nosocomial spread of these infections effectively controlled.
Traditionally, CDI can be diagnosed using the cell cytotoxicity neutralization assay, which detects the presence of toxin, and is often considered to be the clinical gold standard for the detection of C. difficile from fecal samples.8 This method performs with superior sensitivity but is labor-intensive, time consuming (up to 4 days) and requires skilled technicians.9,10 Since the pathogenicity of C. difficile is linked to the two large toxins, toxin A (TcdA) and toxin B (TcdB), toxin enzyme immunoassays (EIAs) have proven to be a significant advancement. These assays are technically simple, fast and frequently used as standalone assays, but have low sensitivity,11 which results in poor positive predictive values if the prevalence of TcdA/B in stool samples is relatively low (<10%).12 Alternatively, the direct detection of genes encoding toxin A and/or toxin B has become a diagnostic target using polymerase chain reaction (PCR) technology. The main advantages of these molecular assays are high sensitivity and relatively short turnaround time.13 However, several drawbacks, including intensive sample preparation, cost and potential for error limit their clinical utility.14 
Recently, significant effort has been focused on the detection of a so-called common antigen for CDI, the glutamate dehydrogenase (GDH) enzyme, which is highly conserved and commonly produced by most isolates of C. difficile in large amounts.15,16 Although GDH assays cannot discriminate between toxigenic and nontoxigenic strains, these assays have been proposed as initial screening tools for C. difficile in stool samples due to their high negative predictive values (>99%).14,17,18 To improve the accuracy and efficiency of CDI diagnosis, two-step or three-step testing algorithms have been utilized involving a preliminary screening via GDH assay followed by a confirmatory test for positive samples using the cell cytotoxin assay, EIAs or PCR for TcdA/B.14,19,20 For example, the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA) guidelines recommend such a two-step algorithm using EIAs for GDH as an initial screening and then the cell cytotoxicity assay or toxigenic culture as the confirmatory test for GDH-positive stool samples.21 
To date, the most commonly used tests for GDH are EIAs, which can provide rapid results, but demonstrate suboptimal clinical sensitivity, ranging from 79.2% to 98%, which varies significantly with the prevalence of CDI.15,22,23 Furthermore, antibodies against GDH could cross-react with GDH produced by other anaerobic bacteria such as Clostridium sporogenes, Peptostreptococcus anaerobius and Clostridium botulinum.24 In addition, the limited stability and high cost of antibodies remain as challenges.
Nucleic acid aptamers are synthetic single-stranded DNA or RNA molecules with a defined tertiary structure that can selectively bind to a target of interest. Remarkable progress has been made in aptamer research since the report of the first aptamers in 1990.25,26 To date, numerous high affinity and highly specific aptamers have been identified against a broad range of targets such as metal ions, small organics, peptides, nucleic acids, proteins, viruses and whole cells.27,28 In particular, aptamers possess several significant advantages over antibodies including low molecular weight, high stability, ease of synthesis and modification, and rapid folding properties.29 Thus aptamers are promising alternatives to antibodies in many different applications, especially bioanalytical applications.30-32 